Abstract

Turbulence induced toroidal momentum transport in boundary plasma is investigated in H-mode discharge using Langmuir-Mach probes on EAST. The Reynolds stress is found to drive an inward toroidal momentum transport, while the outflow of particles convects the toroidal momentum outwards in the edge plasma. The Reynolds stress driven momentum transport dominates over the passive momentum transport carried by particle flux, which potentially provides a momentum source for the edge plasma. The outflow of particles delivers a momentum flux into the scrape-off layer (SOL) region, contributing as a momentum source for the SOL flows. At the L-H transitions, the outward momentum transport suddenly decreases due to the suppression of edge turbulence and associated particle transport. The SOL flows start to decelerate as plasma entering into H-mode. The contributions from turbulent Reynolds stress and particle transport for the toroidal momentum transport are identified. These results shed lights on the understanding of edge plasma accelerating at L-H transitions.

The paper derives the gyro-kinetic equation in the comoving frame of a toroidally rotating plasma, including both the Coriolis drift effect [A. G. Peeters et al., Phys. Rev. Lett. 98, 265003 (2007)] as well as the centrifugal force. The relation with the laboratory frame is discussed. A low field side gyro-fluid model is derived from the gyro-kinetic equation and applied to the description of parallel momentum transport. The model includes the effects of the Coriolis and centrifugal force as well as the parallel dynamics. The latter physics effect allows for a consistent description of both the Coriolis drift effect asmore » well as the ExB shear effect [R. R. Dominguez and G. M. Staebler, Phys. Fluids B 5, 3876 (1993)] on the momentum transport. Strong plasma rotation as well as parallel dynamics reduce the Coriolis (inward) pinch of momentum and can lead to a sign reversal generating an outward pinch velocity. Also, the ExB shear effect is, in a similar manner, reduced by the parallel dynamics and stronger rotation.« less

It is argued that the usual understanding of the suppression of radial turbulent transport across a sheared zonal flow based on a reduction in effective transport coefficients is, by itself, incomplete. By means of toroidal gyrokinetic simulations of electrostatic, ion-temperature-gradient turbulence, it is found instead that the character of the radial transport is altered fundamentally by the presence of a sheared zonal flow, changing from diffusive to anticorrelated and subdiffusive. Furthermore, if the flows are self-consistently driven by the turbulence via the Reynolds stresses (in contrast to being induced externally), radial transport becomes non-Gaussian as well. These results warrant amore » reevaluation of the traditional description of radial transport across sheared flows in tokamaks via effective transport coefficients, suggesting that such description is oversimplified and poorly captures the underlying dynamics, which may in turn compromise its predictive capabilities.« less

Magnetic braking'' of the plasma toroidal rotation in the high confinement H mode by applied resonant, low [ital m],[ital n]=1 static error fields is used in DIII-D [Nucl. Fusion [bold 31], 875 (1991)] as an independent control to evaluate the [bold E][sub [ital r]][times][bold B] stabilization of microturbulence in the plasma core. In the core ([rho][approx lt]0.9) of a tokamak, the radial electric field and its shear are dominated by toroidal rotation. The fundamental quantity for shear stabilization of microturbulence is shear in the velocity of the fluctuations [bold v][sub [perpendicular]][approx][bold E][sub [ital r]][times][bold B]/[bold B][center dot][bold B] which inmore » the core is [ital v][sub [perpendicular]][approx][ital v][sub [phi]][ital B][sub [theta]]/ [ital B][sub [phi]]. With magnetic braking greatly decreasing the toroidal rotation and thus reducing the core radial electric field and shear, far infrared (FIR) measurements of density microturbulence show downshifting in frequency near [rho][approx]0.8 as a result of the reduced Doppler shift ([omega][approx][ital k][sub [theta]][ital E][sub [ital r]]/[ital B][sub [phi]]) and a factor of 2 increase in the turbulence level ([ital [tilde n]]/[ital n])[sup 2] in the period between edge localized modes (ELMs). There is also a large reduction in turbulence at an ELM which tends to compensate for the increase in turbulence with reduced radial electric field shear between ELMs. No significant change is found in H-mode plasma energy, confinement time, internal inductance [ital l][sub [ital i]], density profile, [ital T][sub [ital e]] profile, or [ital T][sub [ital i]] profile. Good H-mode confinement is maintained by the edge ([rho][approx gt]0.95) transport barrier where the reversed edge [ital E][sub [ital r]] and high edge [ital E][sub [ital r]] shear remain unchanged.« less